1. Field of the Invention
This invention relates to a layered structure, such as a printed wiring board or printed circuit board, and a photosensitive dry film to be used as a solder resist or an interlayer resin insulation layer thereof.
2. Description of Related Art
In recent years, in view of high densification of the printed wiring boards to cope with the recent trend of electronic equipment toward decreasing weight and size, the desirability of producing a solder resist having high performance with good workability has been finding growing recognition. Furthermore, in view of miniaturization, reduction in weight, and high performance of electronic equipment, miniaturization of a semiconductor package having a plurality of pins is put in practical use and the mass production thereof is developed. To cope with such high densification of the printed wiring board, the IC packages called BGA (ball grid array), CSP (chip scale package), etc. are recently used instead of the IC packages called QFP (quad flat-pack package), SOP (small outline package), etc. As a solder resist used for such a package substrate or a printed wiring board to be mounted in an automobile, various photosensitive resin compositions are heretofore proposed (for example, see JP 61-243869A).
In packages having a solder resist, since a substrate and a solder resist are heated at the time of sealing an IC chip and driving the IC, they are liable to cause cracks and peeling of the solder resist due to the difference in the expansion coefficient between the substrate and the solder resist. Therefore, for the purpose of suppressing the occurrence of cracks and peeling of the solder resist which are produced at the time of a pressure cooker test (hereafter abbreviated as “PCT”) or thermal cycling, the incorporation of an inorganic filler into a photosensitive resin composition that forms the solder resist is widely performed conventionally so that the linear thermal expansion coefficient of the solder resist corresponds to that of the substrate used as a base of the solder resist as far as possible. However, when a large amount of inorganic filler is incorporated into a photosensitive resin composition in order to make the linear thermal expansion coefficient thereof as low as possible, inorganic filler particles become present in the interface between the solder resist formed and the base, which poses a problem of deteriorating the adhesion of the solder resist to the base.
Further, since an inorganic filler generally exhibits high opacifying effects or ultraviolet absorbing power depending on a material, when a photosensitive resin composition contains a large amount of inorganic filler, there is a problem of decreasing the substantial dose of ultraviolet irradiation to a photosensitive resin and thus easily causing undercure thereof. In order to solve such a problem, there is proposed to prepare a photosensitive resin layer as a two-layer structure; the first photosensitive resin layer containing an inorganic filler being formed on a substrate, and the second photosensitive resin layer that does not contain an inorganic filler being laminated thereon, as disclosed in JP 10-207046A. Such a two-layer structure aims at allowing patterning with a small dose, as compared with the case where only the photosensitive resin layer containing the inorganic filler is patterned as being conventionally performed. That is, since the second photosensitive resin layer will not suffer from the interception or absorption of ultraviolet rays by the inorganic filler, the net dose of ultraviolet irradiation will increase even with the same irradiation dose and the sensitivity as a whole will be seemingly improved.